This invention relates to a method of manufacturing a mask blank and a mask each of which is used in a lithography process on fabricating a semiconductor device or the like and, in particular, to a mask blank and a method pertinent to KrF excimer laser, ArF excimer laser, or F2 excimer laser.
With fineness of a semiconductor circuit and the like, it is a recent trend that requirements have been directed to using an exposure source of a short wavelength in lithography and using lenses of a high numerical aperture (NA) in an exposure apparatus. However, it is to be noted that a high numerical aperture (NA) of the exposure lenses and a depth of focus reside in a trade-off relationship.
In order to attain a depth of focus, flatness required for a photo mask has become severe more and more and should be recently reduced to a range from 0.3 xcexcm to less than 0.5 xcexcm.
In the meanwhile, the flatness of the photo mask depends on bending strength of the transparent substrate, flatness of a substrate prior to deposition of any films, and internal stress of films that form a circuit pattern and the like. Among others, the internal stress of the films has raised a serious problem. Herein, flatness is defined by a difference between a highest point and a lowest one from an average plane determined on a surface used for exposure and can be given by measuring a configuration of a substrate by the use of an optical interferometer. The internal stress is given by the following formula (1):
xe2x80x83Eb2/[6(xcexdxe2x88x921)rd]xe2x80x83xe2x80x83(1)
where E is representative of Young""s modulus; b, a thickness of a substrate; xcexd, a Poisson""s ratio; r, a variable component of a radius of curvature of the substrate; and d, a thickness of a thin film.
In order to satisfy the flatness required for the photo mask, the film for forming the circuit pattern has the internal stress preferably not greater than 5xc3x97108 Pa when use is made of a synthetic quartz substrate of 6 inches square that has a thickness of 0.25 inch and that has films of 70 nm thick.
The internal stress of the films for the circuit pattern is generated during a process of manufacturing a photo mask blank that may be called a substrate with films and that serves to form a photo mask. Taking this into consideration, it has been found out that flatness should be improved in a photo mask blank so as to establish excellent flatness of the photo mask.
Herein, consideration is made about a method of controlling stress of the films for forming the circuit pattern. A method of controlling such stress can include a method of adjusting during a stage of depositing the film and a method of adjusting after deposition of the film.
In the method of adjusting during the deposition stage, a difficulty is present about making adjustment of the stress compatible with keeping characteristics necessary for the photo mask blank. Such characteristics may be an optical characteristic, chemical durability, and the like. Especially, it is often difficult to adjust the stress by deposition conditions in the case where the internal stress is compressive stress.
On the other hand, the instant has already filed Japanese Patent Application No. 2000-277215 and has proposed a method of reducing compressive stress in films for forming the circuit pattern. According to this method, a mask blank is manufactured by utilizing heat treatment carried out after deposition of films.
As any other conventional treatment methods, consideration is made about a method of using a clean oven, a method of using a hot plate, a method of using a lamp heater.
In Japanese Unexamined Patent Publication No. Hei 7-104457, namely, 104457/1995, disclosure is made about a method of manufacturing a half tone phase shift mask. Specifically, the method deposits a translucent film and thereafter forms a stabilization layer on the translucent film. The stabilization layer serves to protect a variation of characteristics in the translucent film that might occur due to irradiating light by a mercury lamp, heating, oxidizing, and due to irradiation of exposure light.
Alternatively, disclosure is also made in Japanese Unexamined Patent Publication No. Hei 8-220731, namely, 220731/1996 about a technique of stabilizing a translucent film of a half tone phase shift mask. According to this technique, the translucent film is irradiated by light, heat, an electromagnetic wave, a particle beam after deposition in an atmosphere of a vacuum, a rare gas, or a reactive atmosphere. In this case, a light source for irradiating light and the like may be, for example, either one of a heavy hydrogen lamp, a Xe lamp, and an excimer laser source, or a combination thereof.
Among the above-mentioned conventional methods, the heat treatment method should heat the transparent substrate to a high temperature not lower than 400xc2x0 C. with the film deposited on the transparent substrate, so as to reduce or alleviate the internal stress of the film. Such a high temperature inevitably lengthens heating and cooling time and lowers productivity.
When heat treatment is carried out by the use of the heat processing apparatus, there is a temperature limit. Consequently, a reduction effect of the internal stress is insufficient for flatness required for the photo mask.
In addition, the light irradiation for the above-mentioned stabilization is not enough to reduce the internal stress because the film can not be heated to a temperature sufficient to alleviate the internal stress.
It is a first object of this invention to provide a method of manufacturing a mask blank, wherein a time needed for improving an internal stress is very short and the internal stress is remarkably improved.
Especially, this invention makes it possible to improve an internal stress of a film that has a difficulty of improving the internal stress and provides the method of manufacturing the mask blank, wherein the internal stress of the film is largely improved.
It is another object of this invention to provide a method of manufacturing the mask blank, which can reduce the internal stress in the film and also improve optical characteristics of the film.
It is still another object of this invention to provide a mask blank or a mask that is not greater than 0.5 xcexcm in flatness.
In order to accomplish the above-mentioned objects, this invention has the following constitution.
(Constitution 1)
A method is for use in manufacturing a lithography mask blank on a transparent substrate. According to this invention, the method comprises the steps of depositing, on the transparent substrate, at least one light absorption film which has a property of absorbing a laser of a predetermined wavelength, and irradiating a laser beam of the predetermined wavelength onto the light absorption film to heat the light absorption film and to thereby alleviate its internal stress.
(Constitution 2)
The mask blank according to Constitution 1 is a phase shift mask blank and the light absorption film is formed by a translucent film which serves as a phase shift film of the phase shift mask blank so as to attenuate exposure light of predetermined intensity.
(Constitution 3)
The mask blank mentioned in Constitution 1 has at least one shield or opaque film and the light absorption film is formed by the opaque film.
(Constitution 4)
In the method according to either one of Constitutions 1 through 3, the laser beam is irradiated onto the mask blank so that the transparent substrate with the light absorption film has flatness not greater than 0.5 xcexcm.
(Constitution 5)
A lithography mask blank according to Constitution 5 is manufactured by the method mentioned in either one of Constitutions 1 through 4.
(Constitution 6)
A lithography mask according to Constitution 6 is manufactured by the use of the mask blank mentioned in Constitution 5.
In order to accomplish the above-mentioned objects, the transparent substrate after deposition of the light absorption film is irradiated by a laser beam in this invention to heat the light absorption film. A stress annealing or alleviating effect according to this invention might result from an effect similar to a heat treatment. However, the laser annealing according to this invention is featured by heating a film for an extremely short time of, for example, several tens of nanoseconds and by heating the film to a highest heating temperature of 1000xc2x0 C. or more. The above-mentioned features bring about a high annealing effect and high productivity that can not be accomplished by the conventional methods.
Specifically, the laser light or beam is transmitted through the transparent substrate but absorbed by the light absorption film. Taking this into consideration, the laser beam of sufficient intensity is irradiated onto the film for such a short time that any damage takes place in the film and heat is not conducted from the heated film to the transparent substrate. In this case, the light absorption film is effectively heated but the transparent substrate is scarcely heated.
Herein, it is to be noted that the thickness of the light absorption film is sufficiently thin in comparison with that of the transparent substrate. Therefore, the light absorption film heated by the laser beam is quickly cooled as compared with the transparent substrate that is not heated after the laser irradiation. Eventually, the temperature of the transparent substrate with the light absorption film is kept substantially invariable during the light irradiation.
The method according to this invention can remarkably shorten the heating and the cooling times and improve the productivity by the above-mentioned effect and is more advantageous in comparison with the conventional method of heating a whole of the transparent substrate with the film by using the oven or the hot plate.
In the method according to this invention, the light absorption film alone is selectively heated by irradiating the laser beam and the resultant transparent substrate is not damaged by the laser irradiation. This makes it possible to sufficiently strengthen the intensity of the laser beam and to sufficiently suppress the internal stress, such as the compressive stress, of the film that may be used for forming a circuit pattern, when the film is formed by the light absorption film. In addition, the internal stress is largely improved in this invention as compared with the conventional heat processing method. This is because the heat treatment is carried out at the temperature which is as high as 1000xc2x0 C. and which can not be accomplished in the conventional heating apparatus.
As mentioned before, irradiating the laser beam onto the transparent substrate with the light absorption film is effective to greatly shift the compressive stress of the light absorption film to a tensile stress side. Consequently, the method according to this invention can easily accomplish desired flatness of the photo mask for a very short time.
Herein, this invention will be described in detail.
It is necessary in this invention to deposit, on a transparent substrate, at least one light absorption film having a property of absorbing a laser beam. In other words, the light absorption films may be composed of a plurality of light absorption layers.
Alternatively, the light absorption film may also be formed by a stacked structure of a light absorption lamina having a light absorption property and an upper lamina that has no light absorption property and a thin thickness. In this case, since the upper lamina having no light absorption property is thin enough, the light absorption lamina is sufficiently heated indirectly via the upper lamina by the laser irradiation. Such absorbing the laser light can be accomplished when the light absorption lamina is located at an uppermost position of either of the transparent substrate or a multi-layer structure or may be placed at a position intermediate between the multi-layer.
The light absorption film which has the light absorption property may be a translucent film used in the half tone phase shift mask or an opaque film in the binary mask or a Levenson phase shift mask. In other words, the light absorption film may be used as a light translucent phase shift film in the half tone phase shift mask blank or an opaque film in the photo mask blank as it is, when exposure is made by using a laser, such as KrF excimer laser, ArF excimer laser, F2 excimer laser, as an exposure light source.
When deposition is made of a plurality of light absorption laminae or a stacked structure of a light absorption lamina and a light non-absorption lamina, the plural laminae may be operable as a light translucent phase shift film in the half tone phase shift mask blank or as an opaque film or a circuit pattern forming film in the photo mask blank.
As a material of the light absorption film operable as the translucent film in the phase shift mask blank, it is possible to exemplify nitrides or nitride oxides of metal and silicon (metal may be, for example, Mo, Ti, V, Zr, Nb, Ta, W, Hf, Cr and etc.), metal nitrides, and/or metals. In this event, the light absorption film is operable not only to absorb the laser beam irradiated on heating the film but also to transmit the laser beam irradiated from an exposure source to some extent. Herein, the light non-absorption film may be formed, for example, by SiO2, Si3N4, Al2O3, CaF2, CrOx, CrFx, and so on.
As regards the phase shift mask blank, it is preferable to carry out heat treatment due to laser irradiation of a laser that has the same wavelength as the laser light source used as the exposure light source. However, it should be considered that high power KrF, ArF, F2 excimer lasers seriously damage an optical system. Taking the above into account, XeCl laser may be practically used which has a wavelength of 308 nm comparatively near to a light source wavelength of the exposure light source.
The light absorption film which as an opaque film in the photo mask blank may be, for example, Si, Ti, V, Cr, Zr, Nb, Mo, Ta, W, Ru, Rh, Hf or the like. In this case, such a light absorption film acting as the opaque film is operable not only to absorb a laser beam used for heating by irradiation but also to shield or shade a laser beam or the like for exposure. This means that the light absorption film has a dense optical density for the laser beam or the like. On the other hand, a light non-absorption film usable for the photo mask blank may be, for example, Si3N4, CrOx, CrFx, or the like.
As for the photo mask blank, the laser irradiation for heating be preferably carried out by the same wavelength as that of the laser source used as the exposure light source but may be carried out by a wavelength which is longer than that of the exposure light source.
In general, the light absorption film according to this invention may be formed by a material that may have an absorption ability of a laser beam. At any rate, such a material may be used for an etching mask film, an antireflection film, or any other films used for any other purposes.
Herein, description will be made about an advantage of this invention in the following. As regards the phase shift mask blank, the conventional heat processing method of heats a whole of a transparent substrate with a translucent film which satisfies optical characteristics as the phase shift mask and can not often sufficiently reduce the internal translucent film of the translucent film. On the other hand, when a high power laser beam is irradiated for a short time in accordance with this invention, the internal stress of the translucent film is effectively and greatly reduced as compared with the conventional heat processing method mentioned above. Output power of the laser beam should be preferably increased within a range of giving no damage onto the film or the substrate. Such high power irradiation can shorten a process time (second unit). In any event, the output power of the laser beam is higher than that of the laser beam used for exposure.
Specifically, this invention can reduce the internal stress of the translucent film to zero or about zero, even when the translucent film is formed by a material that can not reduce the internal stress in the conventional methods. Such a translucent film that can not always reduce the internal stress by the use of the conventional methods may be, for example, nitrides or nitrided oxides of metals and silicon, or a multi-layer composed of a combination of Si3N4, and metal nitrides.
Furthermore, when this invention is applied to the phase shift mask blank, it is possible not only to reduce the internal stress of the translucent film but also to improve the optical characteristics of the translucent film required for the optical characteristics of the phase shift mask blank. Herein, the optical characteristics of the translucent film may be, for example, a desired transmittance and phase angle for exposure light, a required transmittance for test light, chemical durability, irradiation durability, and the like. According to the inventors"" experimental studies, it has been found out that a variation of the transmittance and phase angle due to the laser irradiation of this invention is improved in comparison with the conventional heat processing method. This shows that this invention can improve only the internal stress by the laser irradiation with the variation of the transmittance and phase angle suppressed.
Consideration will further be made about the wavelength of the laser beam in the following. The wavelength of the laser beam may be varied in dependency upon a film material and, therefore, can not be definitely determined. However, the wavelength of the laser beam preferably falls within a range between 157 and 633 nm, and more preferably within a range between 248 and 308 nm. In addition, laser intensity can not be definitely determined because it differs from each material of the translucent film but an energy density may be preferably between 100 mJ/cm2 and 500 mJ/cm2, more preferably between 200 mJ/cm2 and 400 mJ/cm2. At any rate, it is preferable that heating due to laser irradiation may be performed in a vacuum or an inert gas.
As regards the photo mask blank, this invention is also effective to efficiently reduce the internal stress of the opaque film by laser irradiation, in comparison with the conventional heat processing method.
Specifically, this invention can reduce the internal stress of the opaque film to zero or about zero, even when the opaque film is formed by a material that can not reduce the internal stress in the conventional methods. Such an opaque film that has a difficulty of reducing the internal stress by the use of the conventional methods may be, for example, Ta, Si, Nb, Mo, W, and so on.
Like in the translucent film, when this invention is applied to the photo mask blank, it is possible not only to reduce the internal stress of the opaque film but also to improve the optical characteristics of the opaque film required for the optical characteristics of the photo mask blank. Herein, the optical characteristics of the opaque film may be, for example, an optical density, chemical durability, irradiation durability and the like.
As regards the wavelength of the laser beam, it may be varied in dependency upon a film material of the opaque film and, therefore, can not be definitely determined. However, the wavelength of the laser beam preferably falls within a range between 157 and 633 nm, and more preferably within a range between 248 and 308 nm. In addition, laser intensity can not be definitely determined because it differs from each material of the opaque film but an energy density may be preferably between 50 mJ/cm2 and 500 mJ/cm2, more preferably between 100 mJ/cm2 and 300 mJ/cm2.
When the light absorption film is composed of a plurality of laminae, laser irradiation in this invention may be repeated plural times by selecting an optimum laser wavelength and laser intensity.
In the case of the phase shift mask blank, the light absorption film for the laser beam may preferably have flatness not greater than 0.5 xcexcm after laser irradiation of the transparent substrate, and more preferably, not greater than 0.4 xcexcm, 0.3 xcexcm, or 0.2 xcexcm.
Likewise, in the case of the photo mask blank, the light absorption film for the laser beam may preferably have flatness not greater than 0.5 xcexcm after laser irradiation of the transparent substrate, and more preferably, not greater than 0.4 xcexcm, 0.3xcexcm, or 0.2 xcexcm.
Thus, it is possible for this invention to highly improve the internal stress and to enhance an amount of improvement in the flatness to not smaller than 0.3 xcexcm. Such improvement can be established during a very short time (several seconds). Specifically, the flatness of 0.8 xcexcm or more can be achieved for a very short time and can not be accomplished by the heat treatment.
In the phase shift mask or mask blank and the manufacturing the same, no restriction is imposed on the transparent substrate, if the transparent substrate is transparent for an exposure wavelength. Such a transparent substrate may be, for example, a quartz substrate, a fluorite surface, and any other glass substrates or hardened substrates (formed by soda-lime glass, aluminosilicate glass, aluminoborosilicate glass, or the like.
Furthermore, a patterning process (patterning or masking process) of processing the film on the substrate may be carried out by the use of a known lithography technique (photo or electron beam lithography technique) that is used to coat resist, expose, develop, etch, peel the resist, and clean.
Eventually, the laser irradiation may be carried out after deposition of forming patterns.